1. Technical Field
This invention relates generally to methods for producing gyromagnetic resonance and, more particularly, to an improved method and apparatus for providing heterodyne pumping of nuclear magnetic resonance which reduces drive interference and enables observation while exciting a sample.
2. Discussion
Nuclear magnetic resonance is a well known phenomenon in which resonance exhibited by nuclei in a wide variety of atoms reveals information about a sample such as the determination of the molecular structure thereof. This phenomenon has become a valuable tool for use in medical and chemistry laboratory instrumentation devices which include magnetic resonance imaging (MRI) and nuclear magnetic resonance spectrometers. Generally speaking, nuclear magnetic resonance is based upon the existence of magnetic forces or moments attributed to quantized nuclear spins, which when placed in a magnetic field, give rise to distinct nuclear Zeeman energy levels between which spectroscopic transitions can be induced.
It is generally known in the art that when an applied radio-frequency (RF) radiation signal matches the natural transition or resonating frequency for a given nuclei, the resulting interaction is detected as a resonance. That is, nuclear magnetic resonance occurs in the sample when the angular velocity of the RF field equals the angular velocity of the precession of the vector representing the magnetic moment of the sample about the direction of the polarizing field. A more detailed description of magnetic resonance theory is presented in U.S. Pat. No. 2,845,595 issued on Jul. 29, 1958 to Leete entitled "Apparatus For Measuring Electrical Quantities" and which is hereby incorporated by reference.
To induce and observe nuclear magnetic resonance, a number of conventional techniques have been developed. These techniques generally involve placing a sample in a unidirectional direct current (DC) or static magnetic field so as to align the nuclei of atoms making up the sample. One conventional approach for observing nuclear magnetic resonance is commonly known as the Bloch method (or stanford technique) which is based on the well known Bloch equation. According to this approach, a transmitting coil and receiving coil are located near the sample, each having an axis oriented perpendicular to one another and the static magnetic field. The transmitting coil supplies a radio frequency (RF) drive magnetic field transverse to the direction of the static magnetic field so as to induce a nuclear magnetic resonance response from the sample (i.e., pumping the sample). When the RF drive field frequency matches the resonance frequency of the sample, a resonance is produced which is then detected as a voltage induced on the receiving coil.
However, it is often the case that the magnetic field produced from the nuclei may be more than a million times weaker than the drive magnetic field. It is therefore generally necessary to provide for an arrangement whereby the relatively weak signal induced in the receiving coil by the magnetic field of the nuclei is not obliterated or overly interfered with by the strong drive magnetic field. This type of situation in which the drive signal contaminates the received signal is commonly referred to as the drive contamination problem.
According to the prior art, a number of approaches have been used to reduce the drive contamination induced in the receiver coil. One approach is referred to as a time sequencing arrangement which gives rise to a number of pulsed techniques. According to the time sequencing approach, no observation is made of the nuclei while the drive signal is "on" since the nuclear signal would otherwise likely be obliterated due to the strong drive signal induced in the receiver coil. However, it is generally known that the excited nuclei continue to supply a magnetic field for a short interval after the drive signal is turned off. Accordingly, the observation of nuclei by the receiver coil can be made during this interval following the turn off of the drive signal. In practice, there is also a very short interval following the turn off when observation of the nuclear signal may be impossible because the receiver electronics typically do not recover immediately from the strong drive signal.
A second and a third method for reducing drive contamination have commonly been used for continuous wave (CW) or steady state techniques in which the drive signal is applied to the sample at the same time that the nuclei are simultaneously observed. According to the second technique, the drive coil is arranged so that the magnetic field thereof is perpendicular to the axis of the receive coil. In principle, true perpendicularly would eliminate the drive contamination. However, in practice, true perpendicularity requires very precise geometrical adjustment of the drive and receiver coils. In addition, precise adjustments are difficult to maintain in order to minimize the drive contamination to a level where the nuclear signal is usefully observable.
The third technique is somewhat similar to the second approach except a single coil is used in place of the drive and receive coils for providing both the drive and receive operations. According to this approach, a bridge circuit having a compensation coil is used to introduce a signal from the drive source into the output signal so as to cancel the drive signal contamination picked up by the coil. However, this technique also requires a very precise circuit arrangement with a careful adjustment of the component values thereof in order to achieve adequate cancellation of the drive signal associated therewith.
Another method of producing gyromagnetic resonance is described in U.S. Pat. No. 3,462,676 issued to Makoto Takeuchi et al, which provides a method of exposing a sample to an RF magnetic field normal to a polarizing magnetic field while superimposing a modulating audio frequency field on the polarizing field. Magnetic resonance response is detected when the sum of the frequencies of the RF magnetic field and modulating audio field equals the resonating frequency of the sample. This approach, however, requires an RF frequency that is very near the resonance frequency and therefore does not allow for large frequency separations between both of the drive signals and the resonance frequency of the sample. As a consequence, this method is also easily susceptible to drive signal interference and therefore requires rather careful adjustment of the coils in order to minimize the directly induced drive voltage in the receiving coil.
In each of the prior art systems described above, the drive frequency is the same or very near the same frequency as the nuclear resonance frequency. Thus, it is difficult to discriminate between the nuclear signal and the drive contamination picked up by the receiver coil. In the above cited U.S. Pat. No. 3,462,676, the inventors state that many practical difficulties are encountered with higher frequency separations which include direct flux leakage from the modulation coil to the nuclear magnetic resonance receiver coil resulting from the higher power required. It is one goal of the present invention to overcome these difficulties.
It is therefore desirable to provide for an enhanced method and apparatus for producing gyromagnetic resonance in a sample. More particularly, it is desirable to provide for a heterodyne method of producing gyromagnetic resonance which effectively allows for simultaneous observation of the magnetic resonance response from the sample. In addition, it is desirable to provide for such a method which exhibits wide frequency separation between drive signals and the resonance response and is not easily susceptible to drive contamination. It is further desirable to provide for such a method which offers increased measuring capability and the ability to obtain more precise information from the sample.